This invention relates to voltage generating circuits for liquid crystal displays (LCDs) in general, and in particular, to voltage generating circuits that prevent unstable operation of LCDs due to variations in display temperature.
LCDs include a LCD panel that comprises a lower substrate, an upper substrate facing the lower substrate, and a layer of a liquid crystal material interposed between the lower and upper substrates to display an image. The LCD panel is provided with respective pluralities of gate lines, data lines and pixels that are connected to the gate lines and the data lines.
The LCD panels include a gate driving circuit sequentially outputting gate pulses to the gate lines and a data driving circuit outputting pixel voltages to respective ones of the data lines. Each of the gate and data driving circuits is packaged in the form of a chip that mounts on a film or directly on the LCD panel.
Recently, in order to reduce the number of the gate and data driving chips, LCDs have begun to employ a gate-IC-less (GIL) structure in which the gate driving circuit is formed directly on the lower substrate through a thin film process. In GIL type LCDs, the gate driving circuit includes a shift register having a plurality of stages connected to each other in sequential fashion. Each stage is connected to a corresponding one of the gate lines and outputs the gate pulse thereto.
GIL type LCDs have a display characteristic that makes the screen of the display go white when the display is operated at a display temperature that is lower than a normal display operating temperature, and makes the screen go black when the display is operated at a display temperature that is higher than the normal display operating temperature. This is caused by a temperature characteristic of the thin film transistors of the gate driving circuit. That is, the operation of the thin film transistors is hypoactive at low display temperatures and hyperactive at high display temperatures. As a result, operation of the gate driving circuit is unstable because of a variation in the temperature of the display, which in turn, results in a deterioration of the display quality of the LCD.